We present Giant Metrewave Radio Telescope H i observations of an eXtremely Metal Deficient galaxy SBS 1129+576 .
SBS 1129+576 has a weighted mean oxygen abundance of 12 + \log ( O/H ) = 7.41 \pm 0.07 , or 1/18 of the solar value .
Our H i observations show that the galaxy is strongly interacting with a companion ( projected separation \sim 27 kpc ) galaxy , SBS 1129+577 .
H i emission from a third , smaller galaxy , SDSS J113227.68+572142.3 , is also present in the data cube .
We study the H i morphology and kinematics of this small group at angular resolutions ranging from \sim 40 to 8 arcsec .
The low-resolution map shows a bridge of emission connecting the two larger galaxies and a large one-armed spiral distortion of the disc of SBS 1129+577 .
We measure H i masses of \sim 4.2 \times 10 ^ { 8 } , \sim 2.7 ~ { } \times 10 ^ { 9 } , and \sim 2.1 \times 10 ^ { 8 } M _ { \odot } for SBS 1129+576 , SBS 1129+577 and the gas in the bridge , respectively .
Assuming that most of the bridge gas originally came from SBS 1129+576 , approximately one-third of its original gas mass has been stripped off .
The third smaller galaxy has an H i mass of ( M _ { HI } \sim 1.1 \times 10 ^ { 7 } M _ { \odot } ) and does not show any sign of interaction with the other two galaxies .
The higher-resolution maps show that SBS 1129+577 has a central bar and a ring surrounding the bar ; there is also a hint of an integral-shaped warp in SBS 1129+576 .
All these features are very likely to have been induced by the tidal interaction .
In both SBS 1129+576 and SBS 1129+577 , there is , in general , a good correspondence between regions with high H i column density and those with ongoing star formation .
The two brightest H ii regions in SBS 1129+576 have ( inclination-corrected ) gas column densities of \sim 1.6 \times 10 ^ { 21 } and \sim 1.8 \times 10 ^ { 21 } atoms cm ^ { -2 } , respectively .
The inclination-corrected H i column density near the H ii regions in SBS 1129+577 is generally above \sim 2.0 \times 10 ^ { 21 } atoms cm ^ { -2 } .
These values are close to the threshold density for star formation observed in other blue compact galaxies .
In contrast to SBS 1129+576 and SBS 1129+577 which are very gas-rich , the third member of this group , SDSS J113227.68+572142.3 , is gas-poor .